Designed to turn the Twitter interface darker to make it easier to read tweets at night, Night Mode can be enabled by clicking on your profile photo and choosing the "Night Mode" option from the drop-down menu. Turning it off is as simple as clicking the Night Mode setting a second time.
With Night Mode enabled, the traditional light colored interface is made darker with a black overall background and a darker blue background for tweets. Because Night Mode is enabled manually, it will stay activated until it's turned off.
Night Mode has been available on iOS devices since August of 2016 through the main Twitter for iOS app.
Night Mode on the desktop is currently rolling out to Twitter users so you may not see the option under your profile right away.